I did an solvent extraction of $\ce{Eu(III)}$ using $\ce{HDEHP}$ from nitric and hydrochloric acid solutions to determine which acid, and which concentration, will yield the highest distribution coefficient.
My results were interesting, I got that the hydrochloric acid extractions had much lower distribution coefficients, and less dependence on acid concentration (the max distribution coefficient was at 0.3 M). The nitric acid extractions were far more dependent on acid concentration with the global max at 0.01 M and another local max at 0.6 M, as well as being higher in general than the $\ce{HCl}$ distribution coefficients.
I did a literature review, and based on this paper, I think I know the structure of the $\ce{HDEPE/Eu}$ complex. My original complexes were $\ce{EuCl3}$ and $\ce{Eu(NO3)3}$ dissolved in the acid solution, and extracted with 0.8 M HDEHP in dodecane. I know the melting point of solid $\ce{EuCl3}$ is about $\sim\pu{600°C}$ which is far higher than $\ce{Eu(NO3)3}$ ($\sim\pu{85°C}$) so I'm wondering if the stronger bonds in the $\ce{EuCl3}$ complex are preventing the $\ce{HDEHP}$ complex from forming, or if the conjugate bases are somehow participating in the complex formation between $\ce{HDEHP}$ and $\ce{Eu(III)}$.
Does anyone have any insight into my results?